It is well known to those in the art that a tape medium can be used as a data storage back-up subsystem for use with data processing systems since a tape medium has a high data storage capacity. In conventional tape back-up systems a particular quantity of data can be sequentially supplied from a host computer to a tape drive interface unit which in turn supplies the quantity of data in sequence to an associated tape where, accordingly, it is sequentially stored at a current location on the tape. A subsequent quantity of data supplied by a host computer can then be subsequentially stored at an adjacent location following the location of the first quantity of data on the tape. Successive quantities, or batches, of data are likewise sequentially stored on the tape until the end of the tape is reached at which time storage of data on the tape is discontinued and the tape is manually disconnected from the host computer. A new tape is then manually substituted therefor so that further data can be stored on the new tape in the same manner. Data must then be retrieved from each tape separately. An operator selects the correct tape drive and tape combination and connects it to the host computer that is requesting access to the data on the selected tape.
Such a process is relatively slow and requires an operator to manually handle the tape drive and tape units. Moreover, if a tape drive becomes inoperative, access to the data on the tape associated with the inoperative drive cannot be accessed until a new drive unit is installed. Moreover, if some or all data on a tape becomes invalid because of an error therein, such data is not available and no technique is available for recovering the lost data.
It is desirable to be able to operate a tape back-up subsystem without the need for substantial interaction by an operator with the backup subsystem so that the subsystem can remain operable for relatively long time periods when no operator is available, e.g., overnight when such an installation is normally closed. Moreover, such subsystem should permit access to data at all times at relatively high speeds. Further, it is desirable that the subsystem be capable of reliable operation and provide an appropriate technique for recovering data even when an error occurs therein.